1. Field of the Invention
The invention relates to alarms, and more specifically to steering failure alarms for warning an operator of malfunctions in marine steering systems having either linear or nonlinear rudder positioning machinery and either constant or inconstant rate characteristics.
2. Description of the Prior Art
Marine vessels are known to be equipped with steering failure alarms which warn the operator of malfunctions in steering systems having linear rudder positioning machinery and constant rate characteristics. One type of steering failure alarm, for example, disclosed in U.S. Pat. No. 4,055,135 and assigned to the Applicants' assignee, detects malfunctions in steering systems having linear rudder positioning machinery and constant rate characteristics almost immediately after the initiation of a rudder maneuver by utilizing a closed loop simulator. Electrical helm order signals and rudder angle position signals provided by transducers are applied to the closed loop simulator which includes integrating apparatus adjusted to provide a time variant simulated rudder angle signal. The simulated rudder angle signal varies linearly in the same manner that the actual rudder signal would vary in the absence of a malfunction. The simulated and actual rudder angle signals are compared in a summing amplifier whose output is applied to a threshold circuit which activates an alarm whenever the instantaneous values of the simulated and actual rudder angle signals differ by more than a predetermined amount.
The above described apparatus is suitable for use only with steering systems having linear rudder positioning machinery where the static or settled positional accuracy of the rudder is linear with respect to helm position, and having rudder rate characteristics which are constant from one maneuver to the next and which are independent of rudder position. Many actual systems, however, are inconstant in rate characteristics and/or nonlinear in positional accuracy. Some systems, e.g., the Rapson slide, are intrinsically inconstant in rate characteristics because of their geometry, with the variation becoming significant at larger angles, e.g., 30 to 45 degrees. The Rapson slide system is substantially constant in rate characteristics at smaller angles, e.g., 0 to 30 degrees. Depending upon the particular system, the geometry results in the rudder moving at a faster or slower rate at certain angular positions than it does at other angular positions. Moreover, aside from the aforementioned geometric rate inconstancy, a steering system may exhibit rate inconstancy as a result of loose, worn, or misadjusted linkages in the mechanism which controls its power source, e.g., a variable delivery hydraulic pump. Such linkages may cause the rudder rate characteristics to vary between left-going and right-going movements or between successive movements in the same direction. Consequently, when the above-described steering failure alarm is operatively coupled to a steering system exhibiting inconstant rudder rate characteristics, false alarms may result; although the simulated and actual rudders may start from the same position, the inconstancy of the actual rudder's rate characteristics may cause the rudder position, after a certain elapsed time period of a given rudder maneuver, to vary from the simulated rudder position by more than the predetermined value necessary to activate the alarm.
Furthermore, some systems, e.g., Rapson slides where position feedback for the final servo loop is taken from ram position rather than the rudder stock, exhibit static positional nonlinearity. Again, this nonlinearity is most significant at larger, e.g., 30 to 45 degrees, angles. This nonlinearity may cause the actual rudder's static position to vary from the static position of the simulated rudder by an amount which exceeds the predetermined value necessary to activate the alarm.
Accordingly there is a need for an immediate-response type steering failure alarm adaptable for use with all steering systems whether they are constant or inconstant in rate characteristics and whether they are linear or nonlinear with respect to static position.